A mole is the amount of pure substance containing the same number of chemical
units as there are atoms in exactly 12 grams of carbon-12 (i.e., 6.023 X 1023).
This involves the acceptance of two dictates -- the scale of atomic masses and the
magnitude of the gram. Both have been established by international agreement. Formerly,
the connotation of "mole" was "gram molecular weight." Current usage
tends to apply the term "mole" to an amount containing Avogadro's number of whatever units are being considered.
Thus, it is possible to have a mole of atoms, ions, radicals, electrons, or quanta. This
usage makes unnecessary such terms as "gram-atom," "gram-formula
weight," etc.

All stoichiometry essentially is based on the evaluation of the number of moles
of substance. The most common involves the measurement of mass. Thus 25.000 grams of water
will contain 25.000/18.015 moles of water, 25.000 grams of sodium will contain
25.000/22.990 moles of sodium.

The convenient measurements on gases are pressure, volume, and temperature. Use
of the ideal gas law constant R allows direct calculation of the number of moles: n=P
V/R T. T is the absolute temperature, R must be chosen in units
appropriate for P, V, and T. The acceptance of Avogadro's law is inherent in this calculation; so too
are approximations of the ideal gas.